Migrate to the latest Firebase SDK for Swift (v4.0.0)

In version 4.0.0 of the Firebase iOS SDK for Swift, we included changes to follow the
naming conventions in the
Swift API Design Guidelines.
To fix any errors or warnings you might receive when you update your app's
version of the Firebase iOS SDK, follow the steps in this migration guide.

Note: These changes are for Swift only. No naming changes have been made to
the Objective-C SDK.

Changes in the new version

The following changes have been made to the Swift SDK across all Firebase products:

Removing the FIR prefix across names for all constants, protocols, classes,
enums, and type definitions.

Resolve errors

The best way to resolve errors resulting from these changes is to use the Fix-it
dialog in Xcode.

When you open your updated project in your Xcode workspace, errors resulting
from the naming changes appear in the issue navigator.

Click the error and resolve it with the suggestion in the
Fix-it dialog.

Resolve naming conflicts

To resolve naming conflicts, use the namespace provided by the module. For
example, in the case of FIRUser, which is now User, you may want to
differentiate between your User class/struct and the Firebase User class.